Recently, a great deal of attention has been paid to a cellular telephone system of the CDMA scheme in which spread spectrum processing of a carrier wave is performed by using a pseudo random code as a spreading code, and the pattern or phase of the code sequence of the spread signal is changed to allow multiple access.
In a cellular telephone system of this CDMA scheme, the correlation start position of a received spread signal must be determined at high speed. Once this initial synchronization is acquired, control needs to be performed to always maintain synchronization for correlation computation by performing tracing to match a chip timing with path jitter variations caused on a radio channel.
In a general mobile communication system, the influence of multi-path fading poses a serious problem. The CDMA scheme uses a RAKE reception scheme to positively and effectively use such multi-paths. This RAKE reception scheme uses a plurality of fingers for performing reception processing in correspondence with a plurality of multi-paths and searchers for generating reception timings. After reception processing, the resultant signals are combined.
As described above, searchers are used to perform synchronization acquisition of a reception signal and obtain a reception timing from the delay profile of a multi-path.
FIG. 7 shows the concept of general-purpose searchers to explain a conventional search method. Referring to FIG. 7, a reception signal 1 is sent to a plurality of searchers 70 to 72 through a switching unit 2, and one searcher is assigned to each of users 13 to 15. For example, the searcher 70 is assigned to the user #0 (13), the searcher 71 is assigned to the user #1 (14), and the searcher 72 is assigned to the user #2 (15).
FIG. 8 shows the overall arrangement of a receiving apparatus using the conventional search method. FIG. 8 shows only a portion corresponding to one user. Referring to FIG. 8, a radio signal propagating through a radio channel is received by a receiving section 20 through an antenna. This reception signal is converted into a digital signal by an analog/digital conversion section 21 connected to the output stage of the receiving section 20, and becomes a baseband signal. This signal is sent to a multi-path processing section 80. In the multi-path processing section 80, the input digital signal (baseband signal) is sent to a plurality of finger sections 81 which perform processing for each of a plurality of reception paths and a searcher section 82 which generates a reception timing. The finger sections 81 then perform reception processing in accordance with a reception path timing as an output from the searcher section 82.
The reception signal outputs processed by the finger sections 81 are sent to a RAKE combining section 23 to be combined. The resultant signal is sent to a signal processing section 24 to be decoded.
The operation of the conventional searcher section 82 shown in FIG. 8 will be described below with reference to FIG. 9. In the conventional searcher section 82, the entire cell radius is set as a search range 90. An entire delay profile 40 is searched by using the single searcher section 82.
Multi-path in mobile communication has the following two features:
{circle around (1)} Multi-path occurs in a relatively narrow range at considerably frequent intervals. This range does not quickly changes. A search must therefore be made in a narrow range.
{circle around (2)} In mobile communication, an abrupt change in reception state inevitably occurs when, for example, a mobile station moves into the shadow of a building or the like. This phenomenon is called shadowing, which does not occur very often. However, in this phenomenon, it suddenly happens that a signal cannot be received at the preceding reception timing, and the position where a new path will appear cannot be known. For this reason, the entire cell radius must always be searched. That is, the behavior of multi-path in mobile communication includes the above two contradictory features.